1. Field of the Invention
The present invention relates to a coordinate input apparatus for detecting a coordinate of a designated point by sensing vibrations and determining a vibration transmitting time across a vibration transmitting plate.
2. Description of the Related Art
Conventionally, coordinate input apparatus require an operator to input vibrations to a vibration transmitting plate by using a vibrating pen having a piezoelectric element incorporated thereinto. The coordinate of an input point is detected by plurality of sensors provided on the vibration transmitting plate.
In such a coordinate input apparatus, a sensor 6 comprising a piezoelectric element is mounted vertically on the surface of a vibration transmitting plate at the peripheral portion thereof to detect vibrations, as shown in FIG. 11.
However, the waveform detected by the aforementioned conventional sensor structure is a distorted waveform since two modes are superimposed on top of one another causing a deterioration in the detection accuracy for the following reasons.
Vibrations which transmit in a plate-like vibrating member are vibrations, called plate waves, whose transmission speed is determined by the frequency and by the thickness of the vibrating plate. In the plate wave, there are two modes: a symmetrical plate wave (which is mainly composed of a longitudinal wave, shown in FIG. 12(a)) and an asymmetrical plate wave (which is mainly composed of a lateral wave, shown in FIG. 12(b)). In the conventional sensor structure shown in FIG. 11, when the diameter of a piezoelectric element having a vibration mode which is perpendicular to the surface of a vibration transmitting plate 8 (that is, which detects the asymmetrical plate wave mainly composed of a lateral wave) is large as compared with the waveform to such an extent that it cannot be ignored (generally, 1/10th of the waveform or above), the piezoelectric element detects both the asymmetrical wave and the vibrations whose vibration mode is parallel to the surface of the vibration transmitting plate (that is, the symmetrical plate wave mainly composed of a longitudinal wave). Thus, the detection signal from the sensor 6 comprising a piezoelectric element has a distorted waveform in which the two modes are superimposed on top of one another.